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Chapter 4 - Best Management Practices for Erosion and Sediment Control Outlet The outlets shall be designed , constructed and maintained so that sediment does not leave the trap and that erosion of the outlet does not occur. A trap may have several different outlets with each outlet conveying part of the flow based on the criteria below. The combined outlet capacity shall be sufficient for the drainage area . For example , a 12 foot (3 .6 m) earth outlet, adequate for 2 acres (0 . 8 ha) , and a 12 inch (30 cm) pipe outlet, adequate for 1 acre (0 .4 ha) , could be used for a three acre (1 .2 ha) drainage area . There are four types of outlets for sediment traps . Each sediment trap is named according to the type of outlet that it has. Each type has different design criteria and will be discussed separately. r 1 . An Earth Outlet Sediment Trap consists of a basin formed by excavation and/or an embankment. The trap has a discharge point over or cut into natural ground . The outlet width (feet) shall be equal to 6 times the drainage area (acres) . If an embankment is used the outlet crest shall be at least one foot (30 cm) below the top of the embankment. The outlet shall be free of any restriction to flow. The earthen embankment shall be seeded with temporary or permanent vegetation (see Sections 6.65 and 6 .66) within 15 days of construction . (See Plate 4 .25a) ,. 2 . A Pipe Outlet Sediment Trap consists of a basin formed by an embankment, or an excavation and an embankment. The outlet for the trap is though a perforated riser and a pipe through the embankment. The outlet pipe and riser shall be made of corrugated metal. The riser diameter shall be of the same or larger diameter than the pipe . The top of the embankment shall be at least 1 .5 feet (45 cm) above the crest of the riser. At least the top 2/3 of the riser shall be perforated with 1 /2 inch ( 13 mm) diameter holes spaced 8 inches (20 cm) vertically and 10- 12 inches (25 - 30 cm) horizontally. All pipe connections shall be watertight (See Plate 4 .25b) . Select pipe diameter from the following table : Minimum Pipe Diameter in inches (cm) Maximum Drainage Area in acres (ha) 12 (30 cm) 1 (0 .4 ha) 18 (45 cm) 2 (0 .8 ha) 21 (53 cm) 3 ( 1 .2 ha) 24 (60 cm) 4 ( 1 .6 ha) 30 (75 cm) 5 (2 . 0 ha) 3 . A Stone Outlet Sediment Trap consists of a basin formed by an embankment or excavation and an embankment. The outlet for the sediment trap shall consist of a crushed stone section of the embankment located at the low point in the basin . The minimum length of the outlet shall be 6 feet times the acreage of the drainage area (4. 5 m times hectares) . The crest of the outlet must be at least 1 foot (30 cm) below the top of the embankment, to insure that the flow will travel over the stone and not the embankment. The outlet shall be constructed of FDOT No . 1 size crushed stone. (See Plate 4 .25c) A very temporary alternate trap can be constructed from straw bales with a stone outlet. (See Plate 4 .25d) Straw bales shall be installed per Section 4 .05 STRAW BALE BARRIER ( ES BMP 1 .05) . 4-73